Digital image capture devices are becoming ubiquitous in today's society. High-definition video cameras for the motion picture industry, image scanners, professional still photography cameras, consumer-level “point-and-shoot” cameras and hand-held personal devices such as mobile telephones are just a few examples of modern devices that commonly utilize digital color image sensors to capture images. Regardless of the image capture device, in most instances the most desirable images are produced when the sensors in those devices can capture fine details in both the bright and dark areas of a scene or image to be captured. In other words, the quality of the captured image is often a function of the amount of detail at various light levels that can be captured. For example, a sensor capable of generating an image with fine detail in both the bright and dark areas of the scene is generally considered superior to a sensor that captures fine detail in either bright or dark areas, but not both simultaneously.
Thus, higher dynamic range becomes an important concern for digital imaging performance. For sensors with a linear response, their dynamic range can be defined as the ratio of their output's saturation level to the noise floor at dark. This definition is not suitable for sensors without a linear response. For all image sensors with or without linear response, the dynamic range can be measured by the ratio of the maximum detectable light level to the minimum detectable light level. Prior dynamic range extension methods fall into two general categories: improvement of sensor structure, a revision of the capturing procedure, or a combination of the two.
Structure approaches can be implemented at the pixel level or at the sensor array level. For example, U.S. Pat. No. 7,259,412 introduces a HDR transistor in a pixel cell. A revised sensor array with additional high voltage supply and voltage level shifter circuits is proposed in U.S. Pat. No. 6,861,635. The typical method for the second category is to use different exposures over multiple frames (e.g. long and short exposures in two different frames to capture both dark and bright areas of the image), and then combine the results from the two frames. The details are described in U.S. Pat. No. 7,133,069 and U.S. Pat. No. 7,190,402. In U.S. Pat. No. 7,202,463 and U.S. Pat. No. 6,018,365, different approaches with combination of two categories are introduced.